The present invention relates to breathing apparatus of the kind known as powered respirators or power-assisted respirators, in which a motor-driven fan provides a forced flow of filtered air to the face of the wearer to ensure (in normal operation) that he has an adequate supply of clean breathable air when worn in a contaminated environment.
A major benefit to the wearer of a powered respirator, in comparison to a conventional non-powered respirator in which the filter(s) are attached directly to the inlet of the facepiece, is that his lungs are relieved of the strain otherwise caused by inhalation against the resistance of the filter(s). This is particularly so in the case of activated charcoal and the like chemical filters adapted for the removal of contaminant gases and/or vapours from the air, which generally have breathing resistances far in excess of those of dust and other particulate filters.
Most powered respirators are arranged to provide a constant and continuous flow of air to the user irrespective of his actual breathing demand at any particular time. This is, however, quite wasteful and leads to premature exhaustion of the filter(s)--and possibly also of the batteries from which the fan is powered--as it is only during the inhalation phase that the filtered air is actually required. This problem of limited filter life is again particularly acute in the case of gas and vapour filters although it is also true of particle-removing filters.
In order to reduce this problem and effectively increase the useful life of the filter(s) in a powered respirator it has been proposed to control the operation of the fan motor in accordance with a sensed pressure parameter at some point within the apparatus, so that the motor is switched off during exhalation of the user. Air is therefore not drawn through the filter(s) during these periods and the capacity of the filter(s) (and of the batteries for the motor) is not unnecessarily depleted. For example, in United Kingdom patent specification no. 2032284 a sensor monitors the pressure in the facepiece of the respirator and switches off the motor when this rises above a certain threshold as a result of the user's exhalation. In United Kingdom patent specification no. 2141348 a sensor monitors the pressure at a point between the filter(s) and fan (the former being upstream of the latter) and switches off the motor when this also rises above a certain threshold as a result of the user's exhalation (and consequent closure of an inhalation valve at the facepiece).
In these prior systems the pressure in response to which the motor is controlled is sensed as a differential with respect to atmospheric pressure. The pressure sensors accordingly comprise a thin flexible diaphragm communicating on its opposite sides respectively with the interior of the respirator at the chosen point and with the external atmosphere--that is to say the contaminated atmosphere from which respiratory protection is required. It has been found, however, that certain chemical contaminants against which protection may be required can attack or otherwise readily diffuse through the materials from which such pressure sensor diaphragms are conventionally made, with potentially hazardous results for the user. It can also be mentioned that these prior art control systems are essentially full on/full off or two-state systems and hence cannot properly match the delivery of filtered air to the changing breathing demands of the user.